The present invention relates to catalysis, sensors, and therapeutics.
Small metal aggregates show strong catalytic activity, unknown for bulk structures with identical chemical composition. The distinct absorption behavior of metal clusters provides a basis for various optical applications. The characteristic properties of matter in the atom-to-bulk transition range partly result from a strong size dependence of the electronic structure. The discrete energy levels of isolated atoms split and broaden to electron bands in larger aggregates. The band structure determines the propagation and mobility of electrons inside the crystal. In principle, control over the size-dependent electronic structure allows an adjustment of intrinsic material properties to the demands of a wide range of applications.
The high electron density and efficient screening in metals make the critical length scale for the atom-to-bulk-transition considerably smaller than for semiconductors. Gradual development of metallic behavior has been observed for ultra small clusters, either in the gas phase or on surfaces. The transition is characterized by the closure of gaps in the electronic states and the development of collective electronic excitations.
For example, as the metal particle size decreases, the core-level binding energy of metals such as Au, Ag, Pd, Ni and Cu increases sharply. This increase in the core-level binding energy in small particles occurs due to the poor screening of the core-hole and is a manifestation of the size-induced metal-nonmetal transition. Similarly, the interaction of oxygen with silver nanoclusters has shown the ability of the smaller nanocrystals to dissociate dioxygen to atomic oxygen species. Gold nanoclusters on titania are known to catalyze CO oxidation at a cluster size of around 3.5 nm, with the gold behaving more as a non-metal and smaller cluster sizes.
This change is because the average electronic energy spacing of successive quantum levels, δ, known as the Kubo gap is given by 4EF/3n, where EF is the Fermi energy of the bulk material and n is the total number of valence electrons in the nanocrystal. So for an individual silver nanoparticle of 3 nm diameter containing approximately 1000 silver atoms, the value of δ is 5-10 meV. Since the thermal energy at room temperature, kT≈25 meV, a 3 nm particle would be metallic. At lower temperatures, the level spacings become comparable to kT and rendering them non-metallic. Because of the presence of the Kubo gap in individual nanoparticles, properties such as electrical conductivity and magnetic susceptibility exhibit quantum size effects. The resultant discreteness of energy levels also brings about fundamental changes in the characteristic spectral features of the nanoparticles, especially those related to the valence band.
The use of nanoparticles as catalysts has been disclosed in the following: US20040132269, US20040028812, US20040025895, and US20040007241.
A number of applications have been described for the delivery of drugs to targets via the use of nanoparticles (see, for example, U.S. Pat. No. 5,962,566, US20040082521, US 20030152636, US20030064965, and US20020034474).
In U.S. Pat. Nos. 6,281,514, 6,531,703 and 6,495,843 and WO9940628 a method is disclosed for promoting the passage of elementary particles at or through a potential barrier comprising providing a potential barrier having a geometrical shape for causing de Broglie interference between said elementary particles. In another embodiment, the invention provides an elementary particle-emitting surface having a series of indents. The depth of the indents is chosen so that the probability wave of the elementary particle reflected from the bottom of the indent interferes destructively with the probability wave of the elementary particle reflected from the surface. This results in the increase of tunneling through the potential barrier. When the elementary particle is an electron, then electrons tunnel through the potential barrier, thereby leading to a reduction in the effective work function of the surface. In further embodiments, the invention provides vacuum diode devices, including a vacuum diode heat pump, a thermionic converter and a photoelectric converter, in which either or both of the electrodes in these devices utilize said elementary particle-emitting surface. In yet further embodiments, the invention provides devices in which the separation of the surfaces in such devices is controlled by piezo-electric positioning elements. A further embodiment provides a method for making an elementary particle-emitting surface having a series of indents
In U.S. Pat. No. 6,117,344 and WO9947980 methods are described for fabricating nano-structured surfaces having geometries in which the passage of elementary particles through a potential barrier is enhanced. The methods use combinations of electron beam lithography, lift-off, and rolling, imprinting or stamping processes.
In U.S. Pat. No. 6,680,214 a method is disclosed for the induction of a suitable band gap and electron emissive properties into a substance, in which the substrate is provided with a surface structure corresponding to the interference of electron waves. Lithographic or similar techniques are used, either directly onto a metal mounted on the substrate, or onto a mold which then is used to impress the metal. In a preferred embodiment, a trench or series of nano-sized trenches are formed in the metal.
In WO03/083177, the use of electrodes having a modified shape and a method of etching a patterned indent onto the surface of a modified electrode, which increases the Fermi energy level inside the modified electrode, leading to a decrease in electron work function is disclosed. The method comprises creating an indented or protruded structure on the surface of a metal. The depth of the indents or height of protrusions is equal to a, and the thickness of the metal is Lx+a. The minimum value for a is chosen to be greater than the surface roughness of the metal. Preferably the value of a is chosen to be equal to or less than Lx/5. The width of the indentations or protrusions is chosen to be at least 2 times the value of a. Typically the depth of the indents is ≧λ/2, wherein λ is the de Broglie wavelength, and the depth is greater than the surface roughness of the metal surface. Typically the width of the indents is >>λ, wherein λ is the de Broglie wavelength. Typically the thickness of the is a multiple of the depth, preferably between 5 and 15 times said depth, and preferably in the range 15 to 75 nm. FIG. 1 shows the shape and dimensions of a modified electrode having a thin metal film 40 on a substrate 42. Indent 44 has a width b and a depth a relative to the height of metal film 40. Film 40 comprises a metal whose surface should be as plane as possible as surface roughness leads to the scattering of de Broglie waves. Metal film 40 is given sharply defined geometric patterns or indent 44 of a dimension that creates a De Broglie wave interference pattern that leads to a decrease in the electron work function, thus facilitating the emissions of electrons from the surface and promoting the transfer of elementary particles across a potential barrier. The surface configuration of the modified electrode may resemble a corrugated pattern of squared-off, “u”-shaped ridges and/or valleys. Alternatively, the pattern may be a regular pattern of rectangular “plateaus” or “holes,” where the pattern resembles a checkerboard. The walls of indent 44 should be substantially perpendicular to one another, and its edges should be substantially sharp. The surface configuration comprises a substantially plane slab of a material having on one surface one or more indents of a depth approximately 5 to 20 times a roughness of said surface and a width approximately 5 to 15 times said depth. The walls of the indents are substantially perpendicular to one another, and the edges of the indents are substantially sharp.